A telephone line coupler circuit is found in most every type of electronic equipment connected to a telephone line such as modems and fax machines. In order to protect the electronic equipment from surges on the telephone line and side to prevent different ground voltages from causing erroneous operation in the subscriber equipment, transformers or opto-couplers are used in the coupler circuit to connect the subscriber electronic equipment for the telephone line.
Coupler circuits which use opto-couplers are known in the art. In U.S. Pat. No. 4,727,535 to Brandt, a coupler circuit is described in which a single opto-coupler is used for relaying the analog AC transmit signal and another opto-coupler device is used for relaying the received AC signal. The telephone line connect and disconnect circuit (e.g. a line relay) is not disclosed. In U.S. Pat. No. 4,203,006 to Mascia, one opto-coupler is used for relaying a ringing signal to a modem, a second opto-coupler is used in relaying a line seize signal from the modem to the telephone line access coupler and a transformer is used in place of a pair of opto-couplers for relaying the received and transmitted AC signals from the coupler to the modem.
In the known prior art coupler circuits using opto-couplers, the basic functions of relaying the ring signal, relaying the communications receive signal, relaying the line seize signal and relaying the AC transmit signal all require separate opto-coupler devices. In the case that a transformer used, the bi-directional nature of the transformer allows for single device to be used for relaying the received and the transmitted communication signal. In a standard telephone line coupler circuit, the cost of the opto-coupler devices is a substantial portion of the component cost for the circuit.